I've looked everywhere for a few days now, but I can't seem to find the distance between lights on an RLLS approach lighting system. The specific one I am looking at is for the 31C approach at KMDW.
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Approach Lighting Systems (ALS) can have many variations, but each variation will be a combination of certain components.
For example, green runway threshold lights extend across the runway threshold, 5 feet apart (center-to-center), and extend outward 45 feet from the runway’s edge on either side. This line of lights can be up to 10 feet from the landing surface before requiring a waiver.
Centerline lights are rows of 5 lights that span 13.5 feet wide and are usually spaced 100 feet apart extending away from the runway threshold out as far as 3,000 feet in some cases.
Many other lights may be incorporated, including sequenced flashers, runway alignment indicators (RAILS), side row bars, and wing bars.
In general, rows of lights will usually be 100 feet apart for at least the last 1,000 feet, and 200 feet apart beyond that.
For more detail on all of the various types of lighting and specific measurements between each, check out this article on Code 7700.